The invention relates to electrodeposition of molybdenum and at least one other metal on a material adapted to resist the mechanical fatigue to which this support will be exposed under conditions of use, but incapable, on the other hand, of resisting physico-chemical attack (especially corrosion in an acid environment). The material of the support should either be electrically conductive material, or be rendered superficially conductive. Examples of such materials are pure and alloyed iron, ordinary steels, slightly alloyed steels, special steels (stainless steels, "Maraging" steels, etc.), nickel and its alloys ("Inconel", "Monel", "Hastelloy", etc.), copper and cobalt, as well as the respective alloys of these two metals, titanium and metals of the same group, as well as their alloys, and ceramics rendered conductive by a suitable coating (graphite for example).
The good mechanical properties and resistance to hot corrosion which are offered by certain nonferrous nickel base (maximum 65%) and molybdenum base (maximum 30%) alloys, possibly containing chromium, are well known. However, these alloys have not been developed industrially as warranted by their properties and this may be due to their high price and to the difficulties presented in their use.
As a consequence, there is much interest in constituting a part of a material different from such alloys of molybdenum and depositing on this part, forming a support, a coating containing molybdenum which confers superficially on the part the properties of corrosion resistance of the molybdenum alloys.
Attempts were made in the past taking into account, of course, the impossibility of electrodepositing molybdenum alone, in an aqueous medium, on a support of metal or alloy; deposition of molybdenum by such a route is only realizable in association with at least one other metal, called "transition metal", adaptable to electrolytic deposition; such a transition metal can be, especially, iron, cobalt, or, even better, nickel whose association with molybdenum then enables reconstitution, in the electrodeposited layer, of the advantageous characteristics of the non-ferrous molybdenium-base alloys whose high price and difficulties of application have impeded development.
However, it should be noted, and this is of cardinal importance, that the attempts mentioned in the preceding paragraph had a more theoretical objective (verification of the role of the transition metal) than practical, the quality of the alloy layers thus deposited (alloy of molybdenum and transition metal) not having formed the subject of industrial research and development.
The codeposits of molybdenum and nickel, effected in the past by conventional electrolytic methods, have given rise to crude coating layers of little mechanical strength, porous and fissured, these defects being incompatible with any practical industrial application of such a method considering that the essential role of a coating of molybdenum and nickel is to protect, against physico-chemical attack and for as long a duration as possible, the support on which this coating is deposited, which obviously cannot be formed as a fragile, porous and fissured deposit.